1. Field of the Invention
The invention relates generally to pseudolites for satellite positioning systems and more particularly to a pseudolite for transmitting a positioning system signal having identification information corresponding to a positioning system satellite that is currently out-of-view.
2. Description of the Prior Art
The global positioning system (GPS) operated by the United States government uses microwave transmissions from orbiting satellites with known orbits. These transmissions are received by a satellite navigation receiver for determining the location of the receiver. Such microwave frequencies are blocked by the earth or by local obstructions such as buildings. Locating the receiver in an urban city block or inside a building can severely limit performance by excluding necessary satellites from a constellation being tracked.
Each of the GPS satellites transmits a GPS signal on the same carrier frequency modulated by location-determination information from that GPS satellite and spread by pseudorandom (PRN) codes that are distinct for that GPS satellite. Two different PRN codes are used by each satellite: a long code termed the precise/encrypted (P/Y) code and a short code of 1023 bits or chips termed the coarse/acquisition (C/A) code. Either the P/Y code or the C/A code identifies the GPS satellite transmitting the GPS signal and enables a GPS navigation receiver to distinguish the GPS signal from one GPS satellite from the GPS signal from another GPS satellite. The P/Y code is encrypted and restricted for use to those authorized by the United States Department of Defense while knowledge of the C/A code is available to all users.
There are over one thousand distinct C/A PRN codes that could be used for identification for distinguishing the GPS satellites of these C/A PRN codes, the United States government has currently allocated about thirty-two for use by GPS satellites. Existing GPS receivers are designed to search for GPS signals from GPS satellites having any one of these codes. Of these thirty-two allocated satellites, currently about twenty-seven are orbiting and operational and five are not operational. Typically, of the orbiting operational satellites, in mid-latitudes about one-third will be above the Earth horizon and potentially receivable by a GPS receiver and about two-thirds will be unreceivable to the receiver by being below the horizon. Therefore, about twenty-three of the satellites will not be receivable for use by the GPS receiver.
Existing GPS applications use pseudolites to augment the satellite constellation and thus improve availability of the GPS signal. Such pseudolites mimic the satellite transmissions by broadcasting pseudo GPS signals, but are fixed on the ground and transmit the location-determination information appropriate to the geographical location of the pseudolite. The pseudolites make use of PRN codes that have not been allocated for GPS satellites. Signal reception is nearly guaranteed when the pseudolite is located nearby due to relatively higher signal strength of the received pseudo GPS signal. In addition to the thirty-two PRN codes allocated for GPS satellites the United States government currently allocates about four codes for the use of pseudolites. For example, a pair of pseudolites at the end of an airport's runway are conventionally used to enhance the position determination of a navigational receiver in a landing aircraft. It has been proposed that several pseudolites be used in a metropolitan area in order to improve GPS service in urban canyons and inside of buildings.
Fast GPS signal acquisition is important in applications for many GPS navigation receivers. For example, a battery powered receiver alternating between operational and standby modes needs a fast acquisition in order to have good battery life with small batteries. One technique for achieving a fast acquisition is to minimize the number of PRN codes or other types of identifications that are searched in order to acquire GPS signals. However, a requirement for more than four pseudolites increases the number PRN codes that must be stored or generated in a GPS receiver, thereby slowing signal acquisition time in certain circumstances.
There is a need for a pseudolite using a PRN code identification that minimizes the number of PRN codes that must be stored for search in a GPS receiver. Further, there is a need for an autonomous assignment procedure that simplifies and automates the choosing process, especially when there are a multiplicity of pseudolites to be installed, and the pseudolites could be supplied by different manufacturers.